In many power converters employing field effect transistors (FET's), the power FET gate drive circuits need to be electrically isolated from the control circuitry. According to one scheme, an ac voltage is directly coupled through a pulse transformer such that the FET gates are driven with voltages of both polarities. That is, the device is turned on with a positive voltage (e.g., +10 volts) and turned off with a negative voltage (e.g., -10 volts). Hence, a shortcoming of this scheme is that energy is needed to drive the gates in the negative direction as well as the positive direction. As a result, twice the gate drive power is needed as compared with a non-isolated gate driver which drives the gate voltage positive to turn it on and reduces it to zero voltage to turn it off.
Accordingly, it is desirable to provide a gate driver circuit for power FET's which efficiently transformer-couples the gate driver circuit thereto without the need to charge each gate to a negative voltage in order to turn it off and also to reset the flux in the gate drive transformer.